Fuel vapor storage and recovery apparatuses including a fuel vapor storage canister are well known in the art since years. The gasoline fuel used in many internal combustion engines is quite volatile. Evaporative emissions of fuel vapor from a vehicle having an internal combustion engine occur principally due to venting of fuel tanks of the vehicle. When the vehicle is parked changes in temperature or pressure cause air laden with hydrocarbons escape from the fuel tank. Some of the fuel inevitably evaporates into the air within the tank and thus takes the form of a vapor. If the air emitted from the fuel tank were allowed to flow untreated into the atmosphere it would inevitably carry with it this fuel vapor. There are governmental regulations as to how much fuel vapor may be emitted from the fuel system of a vehicle.
Normally, to prevent fuel vapor loss into the atmosphere the fuel tank of a car is vented through a conduit to a canister containing suitable fuel absorbent materials such as activated carbon. High surface area activated carbon granules are widely used and temporarily absorb the fuel vapor.
A fuel vapor storage and recovery system including a fuel vapor storage canister (so-called carbon canister) has to cope with fuel vapor emissions while the vehicle is shut down for an extended period and when the vehicle is being refuelled, and vapor laden air is being displaced from the fuel tank (refuelling emissions).
In fuel recovery systems for the European market normally refuelling emissions do not play an important role since these refuelling emissions are generally not discharged through the carbon canister. However, in integrated fuel vapor storage and recovery systems for the North American market also these refuelling emissions are discharged through the carbon canister.
Due to the nature of the absorbent within the carbon canister it is clear that the carbon canister has a restricted filling capacity. It is generally desirable to have a carbon canister with a high carbon working capacity, however, it is also desirable to have a carbon canister with a relatively low volume for design purposes. In order to guarantee always sufficient carbon working capacity of the carbon canister typically under operation of the internal combustion engine a certain negative pressure is applied to the interior of the canister from an intake system of the engine through a fuel vapor outlet port of the carbon canister. With this atmospheric air is let into the canister to the atmospheric air inlet port to pick up the trapped fuel vapors and carry the same to an intake manifold of the intake system of the engine through the fuel vapor outlet port. During this canister purging mode the fuel vapors stored within the carbon canister are burnt in the internal combustion engine.
Although modern fuel vapor storage and recovery systems are quite effective there is still a residual emission of hydrocarbons let into the atmosphere. These so-called “bleed emissions” (diurnal breathing loss/DBL) are driven by diffusion in particular when there are high hydrocarbon concentration gradients between the atmospheric vent port of the carbon canister and the absorbent. Bleed emissions can be remarkably reduced when it is possible to reduce the hydrocarbon concentration gradient. It is quite clear that this can be achieved by increasing the working capacity of the carbon canister.
However, it should also be clear that only a certain percentage of the hydrocarbons stored in the carbon canister can effectively be purged or discharged during the purging mode. This can be an issue for cars where only a limited time for purging is available, for instance in electro hybrid cars where the operation mode of the internal combustion engine is relatively short.
Another issue arises with the use of so-called flexi fuels which comprise a considerable amount of ethanol. Ethanol is a highly volatile fuel which has a comparatively high vapor pressure. For instance, the so-called E10 fuel (10% ethanol) has the highest vapor generation currently in the market. That means that the fuel vapor uptake of the carbon canister from the fuel tank is extremely high. On the other hand, during normal purging modes of a conventional carbon canister only a certain percentage of the fuel vapor uptake may be discharged. As a result the fuel vapor capacity of an ordinary carbon canister is exhausted relatively fast. The bleed emissions of a fully loaded carbon canister normally then increase to an extent which is beyond the emission values given by law.
In order to improve the purge removal rate during the purging mode few vapor storage and recovery devices have been proposed which use so-called purge heaters. By heating the atmospheric air which is led into the canister through the atmospheric air inlet port the efficiency of removing the hydrocarbons trapped in the micropores of the absorbent is enhanced remarkably.
For instance, U.S. Pat. No. 6,230,693 B1 discloses an evaporative emission control system for reducing the amount of fuel vapor emitted from a vehicle by providing an auxiliary canister which operates with a storage canister of the evaporative emission control system. The storage canister contains a first sorbent material and has a vent port in communication therewith. The auxiliary canister comprises an enclosure, first and second passages, a heater and a connector. Inside the enclosure a second sorbent material is in total contact with the heater. During a regenerative phase of operation of the control system the heater can be used to heat the second sorbent material and the passing purge air. This enables the second and first sorbent material to more readily release the fuel vapor they absorbed during the previous storage phase of operation so that they can be burnt during combustion.
Moreover, the storage canister of the evaporative emission control system according to U.S. Pat. No. 6,230,693 comprises two fuel vapor storage compartments side by side connected by a flow passage. In particular the partitioning of the canister actually means a flow restriction. Because the driving pressure of the flow through the canister is very low it is an important design consideration that flow restrictions be kept to a minimum.